Self-Sacrifice by Males During Mating Shown to be Beneficial

Cannibalism after Mating Beneficial to Species

A recent study completed by researchers at the University of Nebraska-Lincoln and Gonzaga University has shown that self-sacrifice post-copulation by the male, in which the female then consumes him, has benefits for both partners as well as their offspring in the dark fishing spiders. Though they do not yet understand why this happens during mating with this species, the researchers do know that the benefits to both parents and offspring are substantial.[i]

When the females devoured their partner after mating, the offspring were nearly 20 percent larger and survived 50 percent longer than their counterparts in which the mother did not consume the father. This has led the researchers to conclude that cannibalized males benefit through the increased offspring number, size and survivorship.[ii] The male dark fishing spider dies after mating anyway as he has a pedipalp inside him, which is a bulb that suddenly goes off after the male transfers his sperm to the female. The male then curls up and if the female does not consume him, his heart will stop beating within a few hours.[iii]

Since the male is slated to die anyway, the researchers theorize that they could perhaps be making the best out of a bad situation by offering themselves up and providing key nutrients to the female, thereby ensuring that their offspring will reap the benefits by increased survival rates. They also note that it may be futile for the male to try to resist being consumed by the female as she is markedly larger, by almost 90 percent.[iv]

The researchers are also considering why the behavior evolved and believe that the explanation might be a response to first-male precedence. This refers to the phenomenon in which the first male that mates with a female will fertilize more of her eggs than any subsequent males. This is why a male will seek out a virgin for reproduction early in the mating season, even though it means certain death, because it also means that they do not have to compete with other males. Those who survive longer will still encounter more females, but it is less likely that she is a virgin, meaning that he will fertilize fewer of her eggs.[v]

Another interesting finding of the study is also that there is no effect when another protein source is substituted for the male after mating. The researchers tested whether the consumption of a cricket, which is the same size as the male dark fishing spider, would offer the same advantages. Consumption of the cricket, however, did not provide any substantial boost to the survival rates of her offspring. The researchers believe that there may be something unique about the male that provides the nutrient or nutrients that ultimately benefit his offspring.[vi]

This study also seems to contradict past findings about self-sacrifice in the animal kingdom, though it appears that none of the other studies have focused on the dark fishing spider species. They have primarily looked at other spiders and mantids, but in those tests, it was found that cannibalism had little or no effect on the offspring. The researcher pointing this out, though, was also careful to note that the current data is limited and additional studies with a greater diversity of examples would be necessary before drawing any clear and definitive conclusions regarding evolutionary patterns.[vii]